PROJECT SUMMARY This proposal is a component of a P01 program project focusing on the developmental, cognitive, and neural mechanisms of categorization. The P01 as a whole has 4 goals: (1) elucidate ontogenetic and phylogenetic changes in categorization, (2) to link brain development with the development of categorization in humans, (3) to develop and test animal models of category learning, and (4) to use animal models for examining the neurobiology of categorization. These goals will be addressed in three individual projects: (1) Individual Project 1 aims to link the is to link the development and the neurobiology of category learning, (2) Individual Project 2 aims to develop and test animal models of human categorization and category learning, and (3) Individual Project 3 aims to develop and test computational models of interacting systems of category learning. The present project (Individual Project 1) is designed to contribute to goals 1-2 of the P01 program project. Individual Project 1 has the following specific aims: (1) to examine the mechanisms of category learning across development, (2) to conduct a longitudinal investigation of category learning, and (3) to link the development and the neurobiology of category learning. These specific aims will be addressed in two studies with 6-, 8- , and 10-month-old infants, (2) 3- to 4-and 5- to 6-year-old children, and adults. In addition to multiple experiments, we will collect behavioral and biological markers of prefrontal and hippocampal functioning and link these markers to the mechanisms and development of categorization. The behavioral markers will include executive function tasks (these are considered markers of prefrontal functioning) and flexible memory tasks (these are considered markers of hippocampal functioning). The biological markers will include genetic information related to polymorphisms of COMT, DRD4, and BDNF genes, with the former two being markers of prefrontal functioning and the latter one being a marker of the hippocampal functioning. We will use the marker information to predict individual task performance and individual developmental trajectories. The proposed research is based on the general hypothesis of multiple systems of category learning: an evolutionarily primitive system (or systems) based on projections from modality-specific cortices to the striatum plus a more evolutionarily recent system (or systems) involving, in addition to the striatum, cortical areas of the medial-temporal lobe (MTL) and prefrontal cortex (PFC). We also hypothesize that the engagement of each system is determined by (1) the structure of a to-be-learned category, (2) the amount of guidance provided to the learner in a course of learning, and (3) the maturational time-course of each of the systems.